Added a private function that checks if a UTF-8 string is normalized.

* subversion/include/private/svn_utf_private.h (svn_utf__is_normalized): New.
* subversion/libsvn_subr/utf8proc.c (normalize_cstring): New.
  (svn_utf__is_normalized): Implement, using normalize_cstring.

* subversion/tests/libsvn_subr/utf-test.c
  (test_utf_is_normalized): New test for svn_utf__is_normalized.
  (test_funcs): Added test_utf_is_normalized.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1511695 13f79535-47bb-0310-9956-ffa450edef68